The three-level boost converter is the simplest form of the multi-level boost converter in the prior art. Please refer to FIG. 1, it shows the schematic diagram of a typical three-level boost converter. In which, Vin is the input voltage supply, L is the boost inductor, S1 and S2 are the power switches (using MOSFET as an example), D1 and D2 are the power diodes, C1 and C2 are the output capacitors, and R is the load respectively.
While compared with the traditional single-switch boost converter, the three-level boost converter has the following advantages: relatively lower voltage stress on the power elements, higher efficiency, and lower EMI. Therefore, the three-level boost converter is especially suitable for the applications having relatively higher output voltage. After the frequency-doubling technique is employed, the three-level boost converter can be used to lower the input harmonic current and to decrease the current ripples of the boost inductor more effectively.
Though having the afore-mentioned advantages, there is a serious drawback of the three-level boost converter that is the unbalancing of the two voltages across the two output capacitors respectively. This drawback will hamper the real applications of the three-level boost converter.
To overcome the unbalancing of the two voltages across the two output capacitors of the three-level boost converter respectively, the most popular solution in the prior art is to employ the balancing resistors to balance the two voltages. Please refer to FIG. 2, in which, R1 and R2 are the balancing resistors.
The basic operational principles of the above-mentioned voltage balancing technique are described as follows. Under the normal operational conditions, the duty ratios of the switch elements S1 and S2 are different due to the discrepancies of the real circuits and the inconsistencies of the driving circuits and the power elements. The currents flowing through the output capacitors, C1 and C2, are not the same when R1 and R2 are not included so as to result in the unbalancing of V1 and V2. To solve the problem of voltage unbalancing, usually the balancing resistors, R1 and R2, are connected to the output capacitors, C1 and C2, in parallel. Under the circumstances of the voltage unbalancing, the current flows through the output capacitor with a relatively higher voltage will be relatively higher to decrease the unbalancing of the two output voltages so as to achieve the balancing of the two output voltages. Though this alternative is relatively easy to be implemented and has a good effectiveness, but still it has a noticeable drawback that is the power consumption on the balancing resistors are relatively higher.
According to the theoretical derivations, the following result could be reached when R1=R2, and a DC voltage supply is inputted.P*=(ΔD*Vo)/(k*Vin)Wherein, ΔD is the ABS value of the difference between the duty ratios of switch elements S1 and S2, Vo is the average value of the output voltage, Vin is voltage of the input voltage supply, k=|V1−V2/(Vo/2) is a balancing coefficient for measuring the balancing effectiveness, P* is the normalization of the power consumption of the balancing resistors which is the ratio of the total power consumption of R1 and R2 and the input power.
Assuming that the output voltage is 4 times of the input voltage and the difference between the duty ratios of the two switch elements S1 and S2, ΔD, is only 0.1%, if k=0.1 is required, then p*=0.04. Which indicates that under the relatively loose requirement, the power consumption of the balancing resistors are still relatively higher, about 4% of the input power, so as to damper the realization of a higher efficiency and the applications of the three-level converter.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the cross regulation and methods for controlling the booster converter are finally conceived by the applicants.